Polycrystalline super hard materials, such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials.
Abrasive compacts are used extensively in cutting, milling, grinding, drilling and other abrasive operations. They generally contain ultrahard abrasive particles dispersed in a second phase matrix. The matrix may be metallic or ceramic or a cermet. The ultrahard abrasive particles may be diamond, cubic boron nitride (cBN), silicon carbide or silicon nitride and the like. These particles may be bonded to each other during the high pressure and high temperature compact manufacturing process generally used, forming a polycrystalline mass, or may be bonded via the matrix of second phase material(s) to form a polycrystalline mass. Such bodies are generally known as polycrystalline diamond or polycrystalline cubic boron nitride, where they contain diamond or cBN as the ultrahard abrasive, respectively. Examples of diamond and cubic boron nitride abrasive compacts are described in U.S. Pat. Nos. 3,745,623; 3,767,371; 3,743,489; 4,334,928; 5,466,642 and 5,328,875.
For example, U.S. Pat. No. 4,334,928 teaches a sintered compact for use in a tool consisting essentially of 80 to 20 volume percent of high pressure form boron nitride; and the balance being a matrix of at least one binder compound material selected from the group consisting of a carbide, a nitride, a carbonitride, a boride and a suicide of a IVa or a Va transition metal of the periodic table, mixtures thereof and their solid solution compounds. The matrix forms a continuous bonding structure in a sintered body with the high pressure boron nitride interspersed within a continuous matrix. The methods outlined in this patent all involve combining the desired materials using mechanical milling/mixing techniques such as ball milling, mortars and the like.
U.S. Pat. No. 5,466,642 discloses a wear resistant cBN-based cutting tool that includes at least one of a Ti carbide/nitride component, a compound including at least one of Ti and Al, tungsten carbide, Al2O3, and the balance being cBN and incidental impurities. The method of manufacture as described involves wet blending in a ball mill. The incidental impurities mainly result from material abraded from the mill balls and body.
Known methods involving mechanical milling and mixing procedures in order to combine the desired starting materials lead to unavoidable comminution and crushing of said components. This in turn causes a wide spread of particle sizes of the often complex and manifold components to be generated with a resultant lack of homogeneity of the components. This inhomogeneity leads to an inability to accurately determine and control the phase structure of the final material after sintering and in turn the true potential of the material as a cutting tool or the like cannot be exploited. Such materials can also exhibit poor characteristics in applications, which result from an inadequate dispersion and homogeneity of the constituents.
Moreover these procedures are inappropriate as the particle sizes of the desired starting constituents become finer, in particular for submicron particulate materials and more particularly for nano-sized component materials, due to significant difficulties in dispersion. Use of these procedures thus imposes limitations on making composite materials with homogeneous submicron and nano-sized phases.
Further it is impossible to mill ultrahard abrasive particulates without to a greater or lesser extent abrading the mill balls, rods and mill body materials. Larger superhard particle grains lead to more wear of the milling media. The material so generated by this abrasion necessarily contaminates the mix of desired components with either undesirable material or, if that material could be considered as desirable, then it will be introduced in an uncontrollable and variable way. This contamination is particularly prevalent when high energy milling techniques are employed in an attempt to use submicron and nano-sized starting constituent materials. During the life of milling bodies, balls and rods the inescapable abrasion leads to progressive changes in dimensions and surface texture of these items which leads to a progressive change in their milling, mixing and comminution behaviour. These changes lead to further variability in the dispersion, homogeneity, and degree of contamination of the materials being combined and so, in turn, variability in the structure, properties and behaviour in application of the finally produced composite materials and tools. Moreover submicron and nano-grain sized materials are particularly prone to these problems and difficult to make with such methods.
There are examples in the prior art where milling and mixing techniques are not predominantly employed. For example, it is taught in U.S. Pat. No. 5,211,726 that granules of cBN or diamond, of a range of sizes from fine, about 0.1 micron, to coarse, about 1 mm, may be coated in one or more layers of active coating and these coated entities sintered at a pressure and temperature to yield multigrain abrasive compacts. The methods of coating are restricted to chemical vapour deposition (CVD) techniques, for coating multigrained granules of a specific type of cBN material from about 50 micron to about 1 mm in size.
WO 2006/032984 describes techniques in which polycrystalline abrasive elements are produced from abrasive particles. The particles are coated using a colloidal technique, such as sol-gel, with matrix precursor materials. These coatings are heat treated in order to form a chemically protective coating on the surface of the particles.
Polycrystalline cubic boron nitride (PCBN) materials comprise cubic boron nitride (cBN) and a binder material. When used as cutting tools to machine a workpiece, the cBN is prone to chemical wear. This is a particular problem when machining hardened steels. The binder is more chemically resistant than the cBN. It is thought that components of the binder such as TiC, TiN and TiCxNy provide much of the chemical resistance. Coating cBN with phases such as TiC or TiN is expected to improve the chemical resistance of PCBN by acting as a chemical barrier on the outside of cBN particles, but in practice it has been found that such materials do not perform well in machining applications including interrupted tests. This may be because they had not been adequately sintered, and were therefore not sufficiently tough.